In view of expanding networks for the generation and distribution of electrical energy with increasing number of power plants of different types and increased use of renewable energies, methods for storing electrical energy are becoming increasingly important. Pumped storage power plants are considered to be one of the most important, reliable and effective storage systems for electrical energy. In a pumped storage power plant, for example, an upper water reservoir is filled with the use of electrical energy with pumps, whereby the pumping current used is obtained in times of low grid load at relatively low prices. In times of high power demand, the potential energy of the water can then be used by means of a turbine in mechanical rotational energy to drive a generator to generate electrical energy and feed it into the electrical grid. The peak load energy gained can often be re-sold at relatively much higher prices. An efficient and low-loss use of the varying power supply of the electrical grid is of enormous importance.
The generator, which can generate the electrical energy in the turbine operation can also serve as a drive motor for the pumps or pump turbines on the same shaft. Double fed induction machines (DFIGs), which are slip ring rotor induction machines that are fed via both stator and rotor connections, are particularly suitable for motor generator devices for pumped storage power plants. In use, the stator of the induction machine is directly connected to an electrical grid, while the rotor, which is used to control the speed, active and reactive power, is connected to the grid via a frequency converter. This allows oversynchronous as well as undersynchronous operation to the grid frequency and thus the generator speed is variable. Only part of the power has to be adjusted to the desired frequency and power via the converter. To this extent, the converter can be made smaller, less expensive and with lower losses than a comparable synchronous generator, which results in a better efficiency of the overall system. Other advantages of the double fed induction machine over synchronous machines include high operating efficiency under part load, the ability to separately control reactive power and active power, power factor control capability and fast system response time.
A disadvantage of double fed induction machines is that the cyclic heat load of the components increases significantly in an operation with increasing proximity to their synchronous speed. In particular, semiconductor devices that are used in the power electronics of the converter, are prone to considerable junction temperature fluctuations, resulting from the operation of the machine close to the synchronous speed, by which the service life of the converter could be reduced. Since the frequency of the rotor current in a double fed induction machine is determined by the stator flux frequency, thus the grid frequency, and the rotor speed and an operation close to the synchronous speed results in a low rotor current frequency, these rotor currents cause considerable heat load on the semiconductor devices. In this respect, an operation within a predefined deadband around the synchronous speed for the double fed induction machine is to be avoided.
It is known that the maximum temperature of power semiconductors, such as IGBTs and diodes, that are used in the converter increase at lower rotor current frequencies and otherwise constant operating conditions. The lower the rotor current frequency, the higher the fluctuation in the junction temperature of the semiconductors, which reaches peak values when the semiconductor conducts. It is also known that close to the synchronous speed, the power losses are unevenly distributed on the half bridges of the converter.
In order to overcome these problems, various strategies have been proposed to avoid operating the double fed induction machine in a fixed deadband around the synchronous speed or to choose a different, suboptimal operating point. The inadmissible synchronous deadband is specifically determined depending upon the particular system and its components prior to commissioning. Both strategies can lead to considerable efficiency and economic losses for operators of pumped storage power plants. A wide range in which the energy of the electrical grid cannot or not optimally be used, may be unacceptable for operators of pumped storage power plants. This inadequacy therefore must be avoided or at least reduced.
Tan Yingjie et al., “Deadband Control of Doubly-Fed Induction Generator around Synchronous Speed”, IEEE Transactions on Energy Conversion, Volume 31, Issue 4, pages 1610-1621, December 2016, describes a method for preventing an operation within a predefined deadband around the synchronous speed for a double fed induction machine in a wind power plant. It is proposed that a clamping circuit (so-called crowbar) provided for the overvoltage protection, which includes a series circuit of a switching element and a resistor connected between the rotor circuit and ground, be activated in the event of an operation within the predefined deadband to short-circuit the rotor via the clamping circuit. This switches the operating mode of the double fed induction generator (DFIG mode) to a pure induction generator mode (IG mode). To this end, the electromagnetic torque is also adjusted accordingly so that the operating point is shifted from the optimum operating point to a new operating point in the IG mode before the clamping circuit is activated. Thus, a continuous operation, although in a slightly suboptimal operating range, can be ensured.